Broadly, the invention relates to a heat exchanger for cooling a fluid, at a given temperature and pressure, with a second fluid at a lower temperature and higher pressure.
In conventional shell-and-tube heat exchangers, the tube section of the heat exchanger consists of a bundle of tubes which are open at both ends. At each end, the tubes extend through and are welded to a tube sheet. The shell of the heat exchanger completely encloses the bundle. The tubes within the bundle are spaced apart from each other, and from the shell, to define the shell-side section of the heat exchanger.
In a typical heat exchanging operation, one of the fluids (liquid or gas) is passed through the tube section of the heat exchanger. The other fluid is then passed through the shell section, that is, on the the outside of the tubes, usually in a flow path which is countercurrent to the fluid flowing through the tube section. An example of a heat exchanging operation is the cooling of the reaction product from a hydrocarbon cracking furnace. The reaction product is usually a gas which exits from the cracking furnace at a temperature of from about 700.degree.-900.degree. C. As the hot gas leaves the furnace, it is passed through the tube section of the heat exchanger, and cooled by a second fluid, generally water at high pressures, which is passed through the shell section of the heat exchanger. In this operation, part of the heat from the higher temperature gas (reaction product) is transferred through the tube walls to the water. The overall effect is to raise the temperature of the water, sometimes enough to vaporize it, and to reduce the temperature of the gas.
The heat exchangers employed in the cracking process described above have several disadvantages. For example, the deposition of coke on the surface of the tube walls causes fouling of the heat exchanger, and seriously impairs the effectiveness of the heat exchanging operations. When coke build-up occurs, the cleaning operation requires manual washing of the interior of the tubes, as well as a lengthy shut-down of the upstream system. Not only must the cleaning operation be performed frequently, but the heat exchanger can be out of operation for as long as a week. Another problem is the possibility of damage to the heat exchanger tubes due to temperature cycling during cleaning. For example, the heat exchanger must be cooled from a fairly high operating temperature to a fairly low temperature, for the cleaning step and then the temperature must be raised again to resume the heat exchange operation.
Conventional shell-and-tube heat exchangers have been modified in various ways in attempts to improve the efficiency of the heat exchange operation, and/or to reduce the mechanical stresses described above. One of these modifications is described by H. R. Knulle, in "Problems With Exchangers in Ethylene Plants", Chemical Engineering Progress, Volume 68 No. 7 (July 1972), pp. 53-56. This heat exchanger is a double tube construction in which pyrolysis gas, flowing through an inner tube, is cooled by another fluid, which flows through an outer tube enclosing the inner tube. Heat exchangers of this type, as well as many other known exchangers, are difficult to clean, since they require manual washing and a substantial amount of shut-down time. In addition, the exchanger must be cooled prior to cleaning, so that it is not subject to damage from the temperature cycling sequence described above.
The heat exchanger of this invention has several distinct advantages over the known methods and apparatus for exchanging heat between fluids. For example, a fluid at a relatively high temperature can be quickly cooled to a lower temperature. This is a particular advantage in hydrocarbon cracking operations, such as thermal or catalytic cracking reactors, in which the hot reaction product must be quickly cooled to eliminate undesirable by-products. Using the heat exchanger of this invention, the hot reaction product, which generally has a temperature of about 700.degree.-1000.degree. C., can be cooled to below about 500.degree.-700.degree. C. in about 0.03 seconds.
Another advantage of the present heat exchanger is its ability to gradually dissipate relatively high temperatures, which are present in the heat exchanging section, in the direction of the head section, which normally operates at lower temperatures and higher pressures. Dissipation of the heat in this manner eliminates the problem of material degradation, which usually occurs in exchangers where a high temperature zone borders a low temperature zone. Another advantage is that the time required to clean the present heat exchanger is considerably less than that required for conventional heat exchangers. For example, cleaning time generally requires only a few hours. Still another advantage is that the present heat exchanger can be cleaned while it is "on-line", that is, it does not have to be cooled down prior to cleaning. This feature greatly reduces the possibility of thermal degradation of the tubes and other parts of the exchanger from the temperature cycling sequence described earlier.